1) If it's a nightmare or a waking nightmare, then it could be someone from her past or a random image.2) If it's actually occurring, the zombie voice could be Katie hallucinating, as she did when attacked by the guy whose nose she smashed. Corpse may still be just a corpse.3) If it's actually occurring AND she's actually hearing, then Katie is understanding the zombie rasp, an ability we previously only knew of in Charlie as a half-life.

My personal bet is that the answer is #2. There really is a dead guy gutted and drained of blood, but she's hallucinating the conversation.